Prior arts described below are known as examples of an apparatus for separating a material containing conductive particles and insulating (non-conductive) particles by an electrostatic force into the conductive particles and the insulating particles.
Published Japanese Translation of PCT international application No. Hei. 11-509134 (U.S. Pat. No. 5,829,598) discloses a constitution in which a reciprocating insulating mesh conveyor belt is installed between flat-plate electrodes provided with a gap of several millimeters, and by generating friction between the particles, positively charged unburned particles are caused to move toward a negative electrode and negatively charged ash is caused to move toward a positive electrode. This prior art employs friction electrification.
Japanese Laid-Open Patent Application Publication No. Hei. 7-75687 discloses a technique in which dispersed coal ash is dropped to a grounded drum-shaped electrode, thereby separating insulating particles from conductive particles. Specifically, ash (insulating particles) adhere to a rotating drum and unburned particles (conductive particles) are attracted to a high-voltage rod provided in the vicinity of the drum, thereby separating the insulating particles from the conductive particles. This prior art employs induced electrification.
Japanese Laid-Open Patent Application Publication No. Hei. 10-235228 discloses a technique in which particles are electrified by corona discharge and are freely dropped between electrode plates, thereby separating the insulating particles from the conductive particles. This prior art utilizes difference in dropping tracks due to difference in amount of electrified particles.
However, in the prior art disclosed in Published Japanese translation of PCT international application No. Hei. 11-509134 (U.S. Pat. No. 5,829,598), the conveyor belt is adapted to reciprocate in a small gap between the flat-plate electrodes for the purpose of applying friction electrification to the particles. This inevitably results in wear of the belt and the electrode plates, and these components need to be replaced. Therefore, a long-time operation is impossible without maintenance.
In the prior art disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 7-75687, there is no function to disperse powdered material adhering to the rotating drum, which would lead to reduced separating capability due to aggregation.
When much powdered material is fed onto the drum, the layer thickness of the powdered material adhering onto the drum becomes large, which prevents movement of the powdered material on the lower side of the powdered material layer by the electrostatic force, so that separating capability is reduced. Because of the reduced separating capability, the amount of material to be treated is necessarily limited. This makes it possible for a large amount of material to be treated. Also, since the electrode in the vicinity of the drum is rod-shaped, the distance between the powdered material on the drum and the rod-shaped electrode is not constant. An electric field strength varies according to the distance. The separating capability is more degraded in a spot more apart from where the distance is the shortest. In particular, in case of fine powders, the separating capability would be degraded.
In the prior art disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 10-235228, the moving speed of the particles is low because of their free drop. For satisfactory separation utilizing difference in the dropping track, the size of the apparatus need to be increased. Also, in order to perform repeated process for improved separating precision, the apparatus becomes complex and a large amount of material is difficult to treat.
Further, in the above-mentioned prior arts, operating conditions are fixed. Therefore, the separating capability might be sometimes significantly degraded when the particles to be separated have different characteristics.